troubleshooting newly built overdrive/distortion pedals

rocklander · Post by **rocklander** » Sun May 27, 2007 9:50 pm

just built the 'odie' and 'supreaux' from runoffgroove.com and am getting pretty poor results. They're both very quiet, and only really have any response when I attack the strings. Even then significantly more quiet than bypass is.
this site:
http://www.geofex.com/fxdebug/bias_prob.htm
suggests the 'blatty' sound (which I think is what I'm getting is a biasing issue.

question 1: it suggests I diagnose the N-Channel FET (mpf102) with an audio probe, but their other suggestions are somewhat more detailed (like for the PNP or NPN). 'diagnose with the audio probe' doesn't tell me how to.. anyone out there with some advice in this area?
should I try the audio probe on each leg of each FET and adjust the bias sequentially there? if so, looking for what kind of stuff?

question2: I did notice that in biasing the FET I couldn't get the half Rail voltage they suggest unless a signal is present - is that the norm?

question 3: even with a signal present - it's still very touchy and in the supreaux (in its last stage) it's nigh on impossible to get it exact - just SO sensitive to the trip pot... is that to be expected?

any help gratefully appreciated.
cheers

rocklander · Post by **rocklander** » Mon May 28, 2007 12:16 pm

Tulip · Post by **Tulip** » Mon May 28, 2007 8:21 pm

Not my subject, but if you used a plan to build it from, as opposed to a kit, did you use components of exactly the same values as the plan? And, have you checked your wiring?

HTH.

rocklander · Post by **rocklander** » Mon May 28, 2007 8:37 pm

used components that are the same or acceptable equivalents.
ie - instead of j201 FETs I used MPF102 (which are suitable replacements) and instead of a 250KA pot, I used a 500KA pot, which will ultimately just reduce the turning circle of effective resistance.

I've "past participle of get" closer since posting (tonight) and think the j201 replacement ~may~ have changed things, but not significantly. Also the last trim pot may be crackly/dodgy (despite being brand new).
What I did find out was that to bias correctly I needed to ensure all pots following each stage were set open circuit so as to get the correct bias... so far anyway. still working on it, but thanks for the input thus far.

B45-12 · Post by **B45-12** » Tue May 29, 2007 2:02 am

Hi - first check the obvious - battery OK and circuit OK?? - a particular nasty I found recently was the 9volt terminals were making imperfect contact. And that is a known new bettery right - surprising how many have been caught by that nasty - yours truly included.

Assuming cords are OK (check by plugging guitar in directly to amp via cords - both cords not just the one!!!) it does sound like a biasing problem.

So to troubleshoot using an audio probes 1) set all presets and pots to mid points (give them a good few woggles all around as well to make sure they are working correctly - sometimes the metal wiper arm oxidises and needs a twist or two to clean it)and then set the audio probe on the gate and drain of each transistor in turn from no 1 (the input) respectively while playing the guitar (=providing a signal) If you have a sudden volume loss or loose distortion - you have found the stage. if not - do below

Next adjust the 2 pots so gate voltage is about half supply voltage (if you have a digital voltmeter try adjusting them so they are O.5 volts more positive than the source voltage - check also the drain voltage allows a swing of 1-2 volts up to supply voltage - so if supply is 9volts then they should be max 7 volts positive. Cannot achieve any of this?? with trimpots? look at resisitors and check OK with multimeter.

No joy? now look at caps -are the electrolytics all around the correct way - if OK next the caps on the gates of the transisitors - are they OK (try swapping them out/over). Do another check of the wiring.

Last but not least replace trimpots then pots (in case faulty) then if no joy, the trannies (they are after all fets and can/do get damaged easily according to the books - although I've never had problems).

Final desperate act is to strip and rebuild - sometimes you cannot see the wrong connection for love nor money and only find it via this process.

Oh one last thought - you would be surprised how many wiring circuits have 'corrections/misprints' so it may be worth hunting through/about the source you got it from although I cannot see any obvious errors. For example Aussie mag silicon chip usually run a correction 1 2 issues later.

Hope those ideas help - am currently knocking up a copy of it myself so if I find anything I'll let you. know

rocklander · Post by **rocklander** » Tue May 29, 2007 8:22 am

thanks heaps b-45. I've got the odie kinda running - a bit of a hum, but working nonetheless, so defo a biasing issue.

this is the kind of troubleshooting stuff I wanted to hear, so I can now go to town with the DMM and see what the voltages are.
I managed to mostly the supreaux, up to the last stage where it's WAY crackly - am suspecting trip pot on that stage, but will go into it further tonight. The pcb I put together was a pretty shabby job so rewiring and/or removing components is going to be a pain in the arse so I may just have a do-over on a new board (take a bit more time this time).

Last question for now - should adjusting the bias be so temperamental? it's (especially in the last stage) just a gnat's nut moving the trim pot to be at 2.X Volts to being rail (9)Volts.. is that the norm, or perhaps should I be replacing that trimpot?

B45-12 · Post by **B45-12** » Tue May 29, 2007 2:11 pm

Hi - no the bias adjusting chould no way be tempremental or so slight - possilble causes that I can think of 1) defective trim pot or log trimpot 2) dodgy electrolytic on source to -ve rail of transistor, dodgy soldering of trimpot, stuffed tranny. Only once come across stuffed tranny so my thought is
1) check all soldering is OK - no dry joints (easiest to simply go over it again with iron then check with a lens 2) replace trimpot with known LINEAR trimpot (most are), 3) replace cap.

Oooh one more thought check all resisitors associated with transistor who is so tempremental sometimes the body can crack on them - you then get a hairline crack that makes intermittant contact.

Cheers.

rocklander · Post by **rocklander** » Tue May 29, 2007 3:10 pm

ahh... good stuff.. can't wait to get home tonite and have a crack at it.. thanks again B-45.

rocklander · Post by **rocklander** » Wed May 30, 2007 11:22 pm

so I got the response from runoffgroove, and he said that the mpf102 will work, but have a lot less gain thatn the j201. Have now got the j201s from farnell, and have decided to start again from scratch with new trim pots and (more importantly) new pcb cos the first one was pretty much an abortion. should be all up and running again by this weekend and at least then I can bias it all properly (worst mistake was making the board too small and fiddly for my fat arsed soldering iron).
Am confident she'll all go this time.

rocklander · Post by **rocklander** » Thu May 31, 2007 11:18 am

more interesting ideas from the runoffgroove bloke.
the mpf102 is likely to be harder to bias than the j201, so replace the 100K trimpot with 50K or 25K (I'm just gonna put another 100K resistor across it to get the same result.)
He also said that signal is definitely not required for biasing. - just in case that helps @b-45 (or anyone considering building these pedals)

rocklander · Post by **rocklander** » Wed Jun 06, 2007 10:55 pm

update: getting the j201s helped a BUNCH. that and re-etching the board to a more manageable size and better tracks (wasn't using a proper etching pen before dunking it in the ol ferric chloride (sp?), but used a vivid marker - proper pen much better).

next problem - whoa! 100HZ hum.
that cheap ($5 from Tricky Dickies) power supply that BG warned me against worked okay in my bought pedals (must have some power filtering in them) but has the half rectified hum of death - grrrrr!

on a battery the box is great - precisely what I was after

so am now looking at putting a filtering regulating board in the box
based on:

Anyone tried similar? or do I need to regulate and filter further away from the unit. I thought the advantage of doing it in the box is that I can pretty much throw any old/cheap/crap DC adapter that's anything < 30VDc at it and it'll rectify (thru the filters and t7809) down to what the unit needs.

filtering inside the box present any known gotchas people?

B-45 - how'd u get on with your build(s)?

crushing day · Post by **crushing day** » Wed Jun 06, 2007 11:00 pm

B-45 has left the building .......

Post by Bg » Wed Jun 06, 2007 11:03 pm

he's on an enforced holiday for a few weeks....

rocklander · Post by **rocklander** » Wed Jun 06, 2007 11:10 pm

what'd they convict him of?

Jenesis · Post by **Jenesis** » Wed Jun 06, 2007 11:14 pm

Impersonating 1964.